1. Field of the Invention
The present invention relates generally to a positioning and release mechanism between two elements currently used in aircraft actuation systems, and more particularly to a dual load path fail-safe actuation system designed to shift the supported load from the primary load path to the secondary structures at a predetermined force load on the secondary structure.
2. Description of the Related Art
Modern aircraft have horizontal stabilizers located at the rear tail section of the fuselage or the forward section that are pivotally supported relative to the airplane fuselage to “trim” the aircraft during flight by selective adjustment by the operator or auto-pilot from an internal control unit. The stabilizer actuator is a variable length structural link connecting the horizontal stabilizer control surface to the fuselage structure and used to control the pitch (attitude) of the aircraft during takeoff, cruise and landing phases under different aerodynamic loading conditions. The horizontal stabilizer actuator is also used to recover the aircraft during severe aircraft stall situations. In this regard the stabilizer is traditionally connected to the rear section (or tail section) or forward section of the fuselage between a pivot and a position control actuator.
One common trimable horizontal stabilizer actuator consists of a primary ball nut assembly connected with an actuating drive gimbal which is pivotally connected to one end of the horizontal stabilizer structure. The ball nut assembly includes the ball nut housing and is positioned by a rotating ball screw extending axially and usually vertically through the ball nut housing to move a drive gimbal. The ball nut housing is connected to the drive gimbal by means of a trunion, and this translating gimbal is connected to the attachment structure of the horizontal wing. The ball screw, in turn, has one end remote from the actuating drive that contains an extend stop, while a retract stop is located near the driving gearbox and noback assembly which in turn has a gimbal that is fixed from translation or axial movement by a connection to the structure of the vertical stabilizer or fuselage. As the ball screw is rotated, the drive gimbal will be moved in translation relative to it and the fixed attachment. Thus as the ball screw is rotated in one direction, the leading edge of the horizontal stabilizer is pivoted upward, whereas by rotating the ball screw in the other direction, the leading edge of the horizontal stabilizer is pivoted downward achieving the desired horizontal stabilizer angle. Rotation of the ball screw is routinely done by a motor (electric or hydraulic, depending on system architecture) and associated gearing which is connected to the second, fixed support gimbal and which is actuated by the operator or pilot by the internal control unit. The connection of the stabilizer actuator to the stabilizer is located within the vertical stabilizer or fuselage tail section and not directly in the air stream.
The horizontal stabilizer movement, as controlled by the operator or auto-pilot, is transmitted by the ball screw through the actuating drive gimbal by way of the primary ball nut and ball screw plus primary gimbals which defines a primary load path from the wing attachment to the fixed attachment. The movement has a load with a tensile or compressive component as well as a torque component due to the ball screw thread lead. Failures of the primary load path such as caused by the shearing off of the connecting trunion segment, ball screw disconnect or by the loss of nut ball members from the ball nut assembly can result in the complete loss of control of the primary load path, resulting in the loading of the secondary load path. Absent a secondary load path, the failure would be catastrophic to the aircraft. However, stabilizer actuators are normally required to be provided with a secondary load path for alternate support of the stabilizer load. This provides structural integrity as well as meeting the required level of safety. In such structures, the primary load path is normally controllably actuated by the operator or flight computer avionics and is thus under load while the secondary load path is normally designed with gaps that prevent shared loading such that the secondary load path is in a standby mode. In the event of a primary load path failure, the secondary load path is automatically mobilized whereby the stabilizer actuator is jammed in position by means of locks (such as by a tie-rod lock or a secondary inverted nut lock) and rendering the actuator no longer controllable by the operator, pilot or auto-pilot to move the stabilizer. The transfer of support to the secondary load path can occur quite rapidly whereby failure of the primary load path is detected by the operator or pilot by means of the jammed actuator.
The overall design is normally required to leave the engaged secondary load path and jammed actuator with a sufficiently small axial backlash which in the event of repeated load inversions that could enter into an oscillatory mode will not cause deterioration of the wing structure or its connections. Excessive oscillations could cause a catastrophic failure condition for the wing or other aircraft structures. Recent FAA requirements have resulted in additional deflections in the wings attachment between the primary and secondary moving attachments. Because of these new FAA changes, earlier methods, present art, have not been adequate to address this additional deflection during normal operations when the primary load path is intact. The present invention offers a method and solution for allowing sufficiently large gaps between the primary load path and the secondary load path components to prevent load sharing between the two paths during normal operating conditions (primary load path intact). Yet, when the primary load path has failed and the secondary load path is engaged the present bidirectional locking mechanism will trigger minimizing the axial backlash of the secondary load path to allowable levels assuring the actuator in the secondary load path condition is unaffected by an excessive oscillatory mode condition as previously described.